(WIP) Simple flask application

This is a simple flask version of the current application.

The data API needs some more thought.  Currently it is serving the same static files we've been using, but the javascript can access it programmatically by URL rather than filename.  It should be straightforward to get that data from netCDF as in the `vector_animation` script.
my thought was that it would be something like `/data/<source>/step`, but we're not worrying about where the prefetched files came from yet.  We'll also need a data type key here (e.g. velocity, salt, ...).